In machining, such as pressing or injection molding, it is necessary to control a motor while achieving both an operation (i.e., a pressing operation) for controlling the force (pressure) applied to a workpiece and a positioning operation (i.e., positional control) during a non-contact state. Assuming that the workpiece has linear spring characteristics, the pressure applied to the workpiece may be controlled by controlling the pressing amount from the contact point during machining.
Normally, motor control devices have a velocity control loop and a positional control loop. The Publication of Japanese Patent No. 4015139 discloses incorporating a pressing loop that controls the pressing amount as an alternative to the positional control loop so as to achieve both positional control and pressure control.
In order to achieve stable molding quality of the workpiece by controlling the force as designated by a user, it is preferable to increase the gain of a force controller within a stable range. This range is set in accordance with the characteristics of the workpiece and is fixed in a case where the workpiece has linear spring characteristics but is variable in a case where the workpiece has nonlinear spring characteristics. Therefore, in such cases, it is desirable that the gain be changed in accordance with, for example, the pressing amount.
The Publication of Japanese Patent No. 4112571 discloses changing the gain based on a spring constant of a somewhat high value determined from a force command value or a force detection value with respect to a workpiece having nonlinear spring characteristics.